Stability and Noise Performance of Various Rubidium Standards

Among the well-known time and frequency standards from Hewlett Packard and other vendors,
the HP 5065A rubidium vapor frequency standard offers the best medium-term frequency stability of any device that the casual (or even hardcore) "time nut" is likely to encounter.
The Allan deviation of the best commercial cesium-beam standards like the HP 5071A tends to cross into the E-14 range between t=1000 seconds and t=10000 seconds,
while an HP 5065A can reach this level of stability in under 500 seconds. To assess the performance of an HP 5065A fairly, you need an H-maser.

Of course, no conventional rubidium-vapor clock can serve as a drift-free primary standard, so cesium-beam technology was in no danger
of obsolescence when HP announced the 5065A. But now, over forty years later, everyone has access to a perfectly usable (if noisy) primary standard in the form of cheap GPS-disciplined oscillators.
The combined stability of a GPSDO and a quiet Rb standard is hard to beat.

Unfortunately, well-sorted HP 5065As are if anything even harder to find than working cesium standards. Someone who owns a good HP 5065A is probably not interested in selling it.
So it seemed like a good idea to compare the HP 5065A's stability and noise performance to some of the "telecom grade" rubidium standards available from eBay, hamfests, and other venues.

As has long been the case with GPS clocks, the Asian surplus vendors on eBay are selling our own stuff back to us at, well, pretty darned reasonable prices. As of February 2012, ample quantities of FE-5680A
standards are selling for less than $50. They've become popular with experimenters and video bloggers.

The Datum/Efratom LPRO-101 has also been widely
available in the $100-$200 range for the past few years. The LPRO-101s have proven to be excellent performers.
Like many of the other small rubidium standards, they were designed for a long maintenance-free lifespan. Reports suggest that 5-10 years is realistic.

On paper, the PRS10 from Stanford Research Systems seems like the best candidate to
play second fiddle to the HP 5065A, and the short/medium-term stability tests conducted above seem to bear this out. The unit tested here was taken from a Symmetricom TimeSource 2700 CDMA frequency standard
that was never installed. At least when tested with near-zero operating time, it looks like it may actually be competitive with the 5065A I used as a reference in the tests. You can see where the progress of the green trace from the PRS10
halts abruptly as soon as it approaches the HP 5065A's ADEV floor. The two traces remain parallel from t=2000s to near t=10000s, before trailing off into statistical irrelevance. (The red trace was obtained from a different HP 5065A than the one I'm using, but I believe
that their performance is similar.)